The present invention relates to a laser picture-drawing apparatus for drawing a picture on a medium such as a screen, by deflecting a laser beam by use of a deflector and then turning ON and OFF the beam focused, through a focussing lens, on the medium and, more particularly, to a laser picture-drawing apparatus having a function for regulating the focal point position of a laser beam displaced due to a variation in the thickness of a medium.
A conventional laser picture-drawing apparatus will now be described.
A laser beam oscillated from a laser oscillator is deflected (scanned) in a perpendicular direction by deflecting means, and is then focused on the surface of a screen, through a focussing lens.
The beam is bent substantially perpendicularly by a mirror, and then arrives at the screen.
The screen is fastened onto a stage moving on a base, and is moved by a drive mechanism.
Thus, the beam is applied to the upper surface of the screen in accordance with image-forming signals, to thereby draw a picture on the screen.
When the thickness of the screen varies, to thereby cause the focal point position to alter in such a laser picture-drawing apparatus, the focal point position is displaced by varying the position of the lens along the optical axis, and is regulated so that the focal point is matched onto the upper surface of the screen whose thickness, (i.e., height) varies.
According to this device, the deflecting width of the beam on the screen alters due to the variation in the distance between a galvanomirror and the lens. In order to regulate the deflecting width to a predetermined value, it is very complicated to finely alter the deflecting velocity of the galvanomirror.